1. Field of the Invention
This invention relates to an automatic transmission for a vehicle, and more particularly, to an automatic transmission for a vehicle which uses a gear brake structure to fix a predetermined rotational element to a transmission case to achieve a reverse gear mode.
2. Description of Related Art
Generally, in an automatic transmission for a vehicle, a brake mechanism means for fixing a specific rotational element of a transmission mechanism to achieve a reverse gear mode includes a wet multiple-disk type frictional engagement element. Such a reverse gear mode brake mechanism has a relatively large reverse gear ratio, and the transmitted torque is greatly amplified in response to the reverse gear ratio. This requires the brake mechanism, which receives the inverse force (reaction) of the torque, to have a high torque-absorption capacity. Accordingly, the number of disks used in the frictional engagement element becomes large.
When the wet multiple-disk type frictional engagement element is out of engagement, only a very slight gap separates the frictional material from the separator plate to cut off the torque transmission. The gap is very small so that the frictional material and the separator plate can return to the engaging position quickly when required. For this reason, even in the released state, drag torque inevitably is caused due to the induced rotation of the lubricating oil disposed in this gap. Dragging is especially increased, which reduces power-transmissive efficiency, when many disks are used to achieve a high torque-absorption capacity.
Japanese Laid-open Patent Application 6-323377 discloses a structure wherein a specific rotational element is fixed to a case by a gear type clutch mechanism, i.e., a dog clutch, to achieve the reverse mode. The dog clutch and a frictional clutch are positioned in series. The rotational element, which rotates in the torque-transmissive state, is stopped by the frictional clutch through frictional engagement, with the dog clutch still being out of engagement, thereby eliminating dragging.
This apparatus includes a typical transmission mechanism, in which the rotational element is used to achieve both the reverse and forward modes. Thus, the dog clutch and the frictional clutch must be arranged in series. In such a structure, the frictional clutch requires the same torque capacity as a conventional frictional brake. If this combination is utilized as a brake mechanism to achieve the reverse mode, it will need a dog clutch, in addition to the frictional clutch which corresponds to a conventional frictional brake. These additional components prevent the apparatus from being compact.
Thus, the industry lacks an automatic transmission for a vehicle, in which a gear brake is used as a brake mechanism to achieve the reverse mode, in order to substantially eliminate dragging, while allowing the apparatus to be compact.
However, an apparatus wherein the reverse mode brake mechanism is made as a gear brake has the problem that when a specific rotational element is stopped for achieving the reverse mode, that rotational element may still tend to rotate due to the slight rotation induced between rotational elements. This rotation tends to cause gear noise. Therefore, the industry also lacks an apparatus to prevent gear noise due to induced rotation, while allowing the apparatus to remain compact.
Generally, an automatic transmission includes a plurality of engaging elements. The engaging elements are typically driven by a hydraulic servo. Therefore, it is desirable for the gear type brake mechanism to be driven by a hydraulic servo in the same manner as the other engaging elements. The industry lacks a gear brake structure, in which a conventional hydraulic servo is used as the frictional engagement element.
The industry also lacks a compact structure that uses a conventional hydraulic servo, and which prevents gear noise due to the induced rotation described above by operating in the same manner as conventional frictional engagement elements.
Generally, the specific rotational element for achieving the reverse mode in the automatic transmission is one element of a planetary gear. If the rotational element is stopped by a gear brake, the center aligning action of this rotational element may be impeded, in spite of the fact that the optimum centering of the rotational element according to the gearing state with the other gears under the power transmission is essential. Therefore, the industry lacks a gear brake structure for the reverse mode, which maintains the centering function of the predetermined rotational element by modifying the shape of the gear part.
The industry also lacks an apparatus for bringing the frictional engagement element into the engaging position to prevent gear noise prior to bringing the gear member into the engaging position for fixing the predetermined rotational element.
The industry also lacks an apparatus to achieve sequential movement of the frictional engagement element for preventing gear noise, and the gear member for fixing the rotational element, by using a single hydraulic servo.